1. Field of the Invention
The present invention relates to systems and methods for communicating data, and in particular to a system and method for uplinking data using a hybrid multiplexing scheme.
2. Description of the Related Art
Modern satellites often employ a large number of narrow spot beams, often in a beam laydown that forms a cellular coverage of a wide geographic area. In addition to providing better performance, the narrow beams allow spatial re-use of the same frequency or time slot, so that the total throughput bandwidth of the satellite can be several times the allocated frequency band. Because the traffic demand is not equal for all the cells, it would be wasteful to allocate the same amount of bandwidth to each cell. Therefore, satellite systems typically use either Time Division Multiplexing (TDM) or frequency division multiplexing (FDM).
In TDM, each cell is allocated a slice of a “frame” of time, with the allocation repeating once per frame. The TDM approach is best illustrated by the large-terminal TDMA trunking systems such as those operated by INTELSAT. But these systems use regional beams or isolated spot beams, and these beams have much smaller traffic ratios than 100 to 1.
In FDM, each cell is allocated a slice of the total bandwidth. In either TDM or FDM, the allocation is intended to match the communication needs (throughput, etc.) of the traffic in that cell.
The FDM approach can be illustrated by the BEAMLINK system available from COMDEV. The system includes a static switch that directs each incoming cell to one or more of a bank of surface acoustic wave (SAW) filters. Each SAW filter passes a specific band of frequencies. SAW filters are grouped into banks, each of which in aggregate covers the full operating band. Within each bank, the outputs of the SAW filters are combined to obtain a composite signal that spans the full operating band. This signal can then be fed to a digital processor or transmitted to a large Gateway Earth station for demodulation of the individual signals.
The SPACEWAY system provides another illustration of the FDM approach to uplink capacity allocation. The SPACEWAY satellite has a static switch that directs each uplink cell to one or more A/D converters. Each A/D converter accepts a slice of frequencies. For SPACEWAY, the analog-to-digital (A/D) converter output is demodulated and processed on-board the satellite.
In a satellite or stratospheric platform system that covers a wide geographic area with a laydown of overlapping cells, the average traffic in each cell is roughly proportional to the user population within that cell. Cells which cover remote, unpopulated areas generally have far less traffic than cells that cover urban areas. The ratio between highest and lowest traffic cells can exceed 100 to 1. Handling rural cells requires A/D conversion of largely empty uplink bands. Further, because very narrow filters are difficult to implement, satellite systems using FDM have difficulty matching their allocations to such disparate requirements. In contrast, TDM systems can easily allocate very small fractions of time to those (typically rural) cells with light traffic.
This is not a problem for transmissions from the satellite to the ground (downlink transmissions), since the satellite can allocate power among all the cells it is serving, and can use its transmission power for other cells during the rest of the TDM frame. However, the ground station transmitter must be sized according to the instantaneous, or burst, data rate, even though this capability is used only a small fraction of the time. For example, the ground system might require a 100-watt transmitter operating during the 1% of the time that the satellite is “looking at” the cell. If the satellite were “looking at” the rural cell 100% of the time, a 1-watt transmitter could do the same job at much lower cost. However, while the satellite is “looking at” this cell and receiving the low power, lower data rate signal, the satellite cannot use the same assets to receive higher data rate signals from higher power transmitters in other cells. Hence, “looking at” or dwelling on cells with few ground stations transmitting low power signals for extended period of time results in a substantial waste in satellite communication throughput capacity.
Neither BEAMLINK nor the SPACEWAY systems are capable of efficiently handling anything approaching a 100 to 1 ratio of traffic between urban and rural cells. These systems can at best accommodate ratios up to 10 to 1 before they begin wasting capacity on rural cells.
What is needed is a system and method to reduce ground terminal transmitter power requirements, while efficiently utilizing the satellite resource. The present invention satisfies that need.